Abrasive cleaning or cutting

ABSTRACT

An abrasive cleaning or cutting apparatus and method suitable particularly for underwater use at relatively low nozzle (19) overpressures (e.g. conveniently up to about 7 kg/cm 2  above local hydrostatic pressure). The pressurized mixing zone (1) in which abrasive particles from hopper (3), compressed air from air-line (2), and water from water supply line (15) are mixed to form the abrasive stream is arranged such that the abrasive stream includes abrasive particles at least partially surface-wetted by the liquid entrained in air or an air liquid mist as an abrasive carrier. Preferably about 5 to 10% of the liquid is in the mist the remainder going to encapsulate the abrasive particles. The apparatus also includes valves suitably automatically actuable (in response to signals from an underwater sensor) to shut off the surface apparatus from the abrasive-carrying pipeline to restrict or prevent reverse flow in the pipeline should the mixing pressure drop at the surface.

TECHNICAL FIELD

This invention relates to apparatus and methods for abrasive cleaning orcutting.

BACKGROUND ART

Techniques for the underwater cleaning of surfaces have for many yearsrelied principally on the use of manual or powered brushes, scrapers,chisels etc.

More recently, in an effort to improve cleaning efficiency, in suchapplications as the subsea cleaning of welded regions of metalstructures prior to safety testing or inspection where high standards ofcleaning are demanded, blast cleaning systems employing a high pressurejet of an abrasive slurry have been tried, using water as a carrier forthe abrasive. However, the use of such slurries has been found topresent many difficulties. For effective cleaning action, the slurrymust emerge from the nozzle at a pressure of at least 2000 psig (141kg/cm²) above the local hydrostatic (ambient) pressure, more typicallyfrom 7000 to 15000 psig (490-1060 kg/cm²) above the hydrostaticpressure. As well as the need for expensive pumping equipment andcomponents capable of withstanding the very high delivery pressuresrequired, large reactive forces are generated at the nozzle, causingdifficulty in orientation and manipulation, and considerable danger tothe diver operating the equipment. Furthermore, the equipment is proneto very high degrees of internal abrasion from the high pressure slurry.

DISCLOSURE OF THE INVENTION

The present invention is based in one aspect on the finding that bypreparing the abrasive stream in a particular manner described in moredetail below, a method and apparatus can be achieved permitting faster,safer, and more effective cleaning at relatively low nozzle pressures.Indeed we have found that the action of the abrasive stream can be soeffective that the apparatus can be employed for the purpose of abrasivecutting of materials.

According to a first aspect of the present invention, there is providedan apparatus for abrasive cleaning and/or cutting, suitable particularlybut not exclusively for underwater abrasive cleaning and/or cutting,which comprises a mixing zone for preparing an abrasive mixturecomprising abrasive particles, air (the word "air" herein including alsoother gases) and a liquid, an outlet nozzle for directing a stream ofthe mixture at a surface to be cleaned and/or cut, a pipeline connectingthe mixing zone to the outlet nozzle for conveying the abrasive streamto the nozzle, and means for supplying the abrasive particles, air andliquid to the mixing zone in such a way that the resultant abrasivestream includes abrasive particles at least partially (preferablysubstantially) surface-wetted by the liquid and entrained in air or anair/liquid mist as an abrasive carrier.

The invention further provides a method of abrasive cleaning and/orcutting in which an abrasive stream comprising a mixture of abrasiveparticles, air and a liquid is directed under pressure at a surface tobe cleaned and/or cut, the abrasive stream including abrasive particlesat least partially surface-wetted by the liquid and entrained in air oran air/liquid mist as an abrasive carrier.

It is desirable that the abrasive stream leaves the mixing zone insubstantially the form of a fine mist as a propellant entraining theabrasive particles. It is preferred that the mixing is carried out underpressure. The mist and surface-wetted particles may suitably be obtainedby the accurate control and metering of proportions of abrasiveparticles, air and liquid, to achieve reduced resistance to outflow ofthe abrasive stream, and a greatly enhanced performance.

Given suitable control of ingredients supplied, the required abrasivestream can be achieved without the need for an atomiser (whereby theliquid would enter the mixing zone in atomised form). The word "atomise"in the context of this invention refers to the formation of liquiddroplets of sufficient size to wet the abrasive particles.

Without wishing to be bound by theory, it is believed that undersuitable conditions, in the present invention the abrasive particlesthemselves can break up the liquid in the abrasive stream to create therequired effect. Factors effecting the dispersal or atomisationconditions in the abrasive stream may include abrasive particle size,depth of operation, abrasive stream flow rate and nozzle pressure.

In more detail, the apparatus suitable provides for the liquid toimpinge on the pressurised air/particle stream as initially a continuousliquid stream (i.e. without atomisation), whereby the effects of theparticle stream and the inevitable particle turbulence cause the liquidstream quickly to break into droplets somewhat larger than the size ofthe abrasive particles themselves. To permit the necessary wetting, themixing zone must be of sufficient length and sufficient effective volumeto permit further breaking of the said droplets (due to the particleturbulence, the droplet turbulence and to mechanical effects of themixing zone configuration, particularly the effects of the mixing zonewalls, the junction with the liquid inlet port, etc.) to proceed to astate where the liquid droplets are substantially the same size as theabrasive particles. We have found that at this size the necessary degreeof wetting is optimised.

By controlling the liquid supply rate to ensure that suitably only abouta 5 to 10% surplus of liquid above that needed to wet the particles, andusing a mixing zone of sufficient length as described above, theabrasive stream can be readily prepared.

The mixing zone suitably comprises a length of rigid tubing, into oneend of which is introduced compressed air at a suitable volume andpressure. The mixing zone should have a similar internal diameter tothat of the pipe introducing the compressed air, so that the velocity ofthe air stream is maintained. The mixing zone is preferably relativelyelongated, to permit the air flow and the air-entrained abrasiveparticle flow to merge before contacting the liquid flow, which ispreferably introduced at an angle to the air/particle flow.

The mixing zone should preferably possess an effective volume forenabling the ingredients to form an abrasive stream in which theabrasive particles are at least partially wetted, e.g. approximately80-100% (suitably 90-95%) of the liquid encapsulating the abrasiveparticles, and the remainder, if any, of the liquid forming a fine mistat discharge.

For underwater use, the liquid mist should preferably contain no morethan 10% of the liquid used, as a greater amount of mist has been foundto impede the abrasive flow and to reduce the effect of the abrasive atthe surface to be cleaned.

The mixing zone preferably has one top connection (from a pressurisedabrasive hopper or vessel) through which abrasive particles areintroduced into the air stream in a carefully and precisely regulatedamount in proportion to the volume/pressure of air by suitable valvemeans as described below; then one further connection preferablydownstream of the abrasives connection through which a liquid isintroduced, using a variable volume positive displacement or meteringpump, or control valve means, or both, to accurately control the volumeof liquid thus introduced.

The top (abrasive inlet) connection should be as close to the air inletconnection as is practical, e.g. about 4" to 6" (100 mm to 150 mm),where natural turbulence of the air stream will create maximum agitationof the abrasive particles. The liquid may then be introduced at anyconvenient downstream point in the mixing zone as a continuous stream orjet and without necessarily using any special form of atomising nozzle,as it has been found that the combination of turbulence and impact withthe abrasive particles travelling at high velocities within the airstream proves to be an adequate dispersant of the liquid into a finemist, at the same time ensuring a thorough wetting (or encapsulating ina liquid film) of the abrasive particles, which is the effect which itis desired to obtain to achieve the optimum performance underwater.

The volume of the liquid introduced is thus equally important inproportion to the air volume as is the quantity of grit. Too littleliquid and the air stream will remain dry, or some of the abrasiveparticles will remain dry, thus losing considerable efficiency andgreatly increasing wear within the apparatus. Too much liquid and acushion will be created between the abrasive particles and the surfaceto be cleaned or cut. With careful control this feature can be usefullyemployed, for example where only partial removal of a coating orcontaminant is required.

If the liquid is introduced into the mixing zone prior to the abrasivesthe effect is much the same, but the dispersal of the liquid and itssubsequent atomisation by the abrasive impact is less, therefore such amethod is less efficient unless additional dispersant means, such as aspray or atomising device, is used. Also, more operator care is neededin order to avoid a build up of damp abrasive in the mixing chamber,when such an arrangement is used.

To maintain an even homogeneous mix desirable for maximum efficiency,the discharge orifice from the mixing zone should be about the samediameter as the air inlet, and the delivery pipe to the cleaning nozzleshould have a similar internal diameter as that feeding the air into themixing zone.

Filtration means may be incorporated into the air/gas supply pipe toremove entrained oil and moisture, as dirty air will have an adverseeffect on efficiency and in the extreme could cause blockages.

The liquid will most suitably be clean fresh or sea water. Forunderwater cleaning or cutting the liquid will normally be the samemedium as that in which the operation is carried out. Other liquids may,however, be used if desired, in which case for underwater use theyshould desirably have a surface tension and viscosity approximatelyequal to the water in which the operation is being carried out. We havefound that in some circumstances performance of the apparatus can beenhanced if the liquid is heated before passing to the mixing zone (e.g.hot water may be used).

The abrasive particles may be selected from sand (e.g. sharp sand),grit, copper slag or other conventional material. The abrasive should beof good quality, dry and clean, and typically of mesh size 16-30. Theparticle sizes suitably range from about 0.02 mm to 2.50 mm diameter forunder-water work, typically a mix within the range 0.6 to 1.5 mmdiameter). Preferably the abrasive particles will be entrained in astream of compressed air prior to entry to the mixing zone, and passedto the pressurised mixing zone through a single inlet thereof. Means maybe provided for assisting a smooth flow of abrasive particles to themixing zone during operation by the introduction of relatively highpressure air into the abrasive particle supply system.

It has been found that the mixing conditions of the present inventionenable a homogeneous mix of air, water and abrasive to be obtained. Thisis believed to contribute to the considerably enhanced performance andthe effectiveness in underwater use of a much lower nozzle pressure,typically less than 100 psig (7 kg/cm²) (e.g. normally between about 20and 50 psig (1.4 to 3.5 kg/cm²) above local hydrostatic pressure forcleaning purposes and between about 30 and 80 psig (2 to 5.5 kg/cm²)above local hydrostatic pressure for cutting purposes), compared withthe high nozzle pressures of known systems. Without wishing to be boundby theory, it is believed that when substantially all of the abrasiveparticles in the stream are wetted over their surfaces in a morethorough and efficient way than available hitherto, this gives a greatlyreduced resistance to flow through water after leaving the nozzle, evenat extremely low nozzle pressures. It is also believed that, due to therelatively low impact velocity on the surface to be cleaned or cut,there is a relatively very low reactive force; consequently, in cleaningoperations where the abrasive stream is applied across the surface to becleaned, the surface tension of the liquid film encapsulating eachabrasive particle is believed to cause it to cut across the surface ofthe object to be cleaned, rather than bouncing off, thus achievingmaximum utilisation of the kinetic energy of the abrasive stream.

As mentioned above, the supply of the components of the abrasive streammust be carefully controlled. Typically the apparatus of the inventionmay have the following specification:

Particle flow rate: 0.25 to 4.0 kg/min, suitably 2.0 kg/min.

Liquid flow rate: 0.25 1/min to 10 liters/min, suitably 2 1/min.

Air flow rate: 600 to 1350 m³ /hr.

Mixing zone volume: 120 to 500 cm³, suitably 250 cm³.

Mixing zone pressure: typically about 3.5 kg/cm² above hydrostaticpressure at the nozzle.

The air flow rate and mixing zone pressure will depend on the workingdepth in underwater use. According to the invention the adjustment maybe manual or automatic. The liquid flow rate may also be adjusted asdesired, automatically or manually as described below. The above-quotedfigures are typical for working down to underwater depths of about 400ft (122 m); for greater depths certain figures will correspondingly bechanged, as readily understandable to those skilled in this art.Particularly preferred figures for compressed air supply pressures andflow rates are given in Table 1 below:

                  TABLE 1                                                         ______________________________________                                        AIR COMPRESSOR RATES                                                                       MINIMUM                                                                       COMPRESSED     MINIMUM                                                        AIR SUPPLY     COMPRESSOR                                        WORKING DEPTH                                                                              PRESSURE       CAPACITY                                          FEET   METERS    P.S.I.G. KG/CM.sup.2                                                                           C.F.M.                                                                              M.sup.3 /HR                           ______________________________________                                         50    15        100      7       350   595                                   100    30        100      7       350   595                                   150    46        125      8.8     400   680                                   200    61        150      10.6    450   765                                   250    76        175      12.3    500   850                                   300    92        200      14.1    550   935                                   350    107       225      15.8    600   1020                                  400    122       250      17.6    650   1105                                  ______________________________________                                    

In its application at relatively low nozzle pressures, the method andapparatus of the invention provides a scouring, rather than blasting,action on the surface to be cleaned, unlike underwater cleaning methodshitherto known. Typically, the homogeneous abrasive mix prepared in thepresent invention is propelled across as well as onto the surface,acting to undercut as well as abrade the coating or contaminant to beremoved. In this way, we have found that trapped contaminants can bereleased from cracks, crevices and pits in surfaces, leading to a muchcleaner finish than previously attainable.

In the case of underwater abrasive cutting of materials, we have foundthat conventional pipeline casings, bindings or coatings such as thosecomposed of concrete or synthetic materials can be cut through safelyand efficiently using the apparatus, preferably employing a nozzledischarge pressure of around 30 to 80 psig (2 to 5.5 kg/cm²) above localhydrostatic pressure, (i.e. generally slightly higher than for abrasivecleaning applications).

It is a normal requirement of underwater abrasive systems that thedischarge of the abrasive stream into the pipeline should be stoppableat the surface on the command of the nozzle operator. When working atdepth, however, once the stream is stopped the pressure within themixing zone would normally drop to atmospheric as the grit vesseldepressurises. Since the hydrostatic water pressure surrounding theflexible discharge pipeline increases substantially with depth, thiswill cause an accelerating reverse flow of the abrasive mix back throughpipeline which could create a syphonic effect flooding the apparatus onthe surface.

The present invention includes in a second aspect an abrasive systemdesigned to avoid such difficulties.

According to a second aspect of the present invention, there is providedan apparatus for underwater abrasive cleaning and/or cutting, whichcomprises a mixing zone for preparing an abrasive mixture comprisingabrasive particles, air and a liquid, an outlet nozzle for directing astream of the mixture at a surface to be cleaned and/or cut, and apipeline connecting the mixing zone to the outlet nozzle for conveyingthe abrasive stream to the nozzle, wherein valve means are providedupstream and/or downstream of the mixing zone actuable to restrict orprevent flooding of surface apparatus due to reverse-flow of abrasivemixture in the pipeline.

The valve means are preferably actuated in response to local hydrostaticpressure at the nozzle, most preferably via automatic actuatorscontrolled by a signal from the nozzle, but may equally effectively bemanually actuated by the machine operator in response to such signal orother indication of pressure loss (at the nozzle) or reversed pressuredifferential between the nozzle discharge pressure and the localhydrostatic pressure, whereby the local hydrostatic pressure becomesgreater than the pressure either at the nozzle or the mixing zone.

The valves may suitably each comprise a resilient tube snugly retainedunder longitudinal compression within a chamber and seated therein byexpansion against abutments provided in the chamber, the arrangementbeing such that the respective flowable medium may pass through the tubein use and means being provided for wholly or partially constricting thetube, wherein the abutments in the chamber are so shaped that at leastpart of the surface against which the tube is seated faces away from theaxis of the tube.

The shape of the abutments causes the radially inner part of the tubewalls to be generally more longitudinally compressed than the radiallyouter part, and also causes a reaction force to act on the tube walls ina direction away from the axis of the tube. Since the security ofseating of the tube within the chamber is dependent on the direction andforce with which the seated portions (e.g. the ends) of the tube wallsand the abutments bear against one another, the valve constructioneffectively reduces the danger of unseating the tube even at relativelylow degrees of longitudinal compression. The low degrees of longitudinalcompression can allow buckling of the tube into the fluid flow path tobe minimised, so lowering the amount of wear of the tube inner surface.

Known resilient tube valves also suffer from the disadvantage that theycannot be pre-set at a desired minimum and/or maximum constriction.

In a further aspect, therefore, the invention provides a valvecomprising a resilient tube snugly retained under longitudinalcompression within a chamber and seated therein by expansion againstabutments provided in the chamber, the arrangement being such that aflowable medium may pass through the tube in use and means beingprovided for wholly or partially constricting the tube, wherein the saidmeans for constricting the tube may be pre-set to provide a desireddegree of constriction of the tube when actuated.

In a preferred form, the constriction means may comprise two nip headsarranged to bear against opposite sides of the tube to squeeze orrelease the tube by mutual respective closing or opening. One of the nipheads may suitably be manually adjustable and the other remotelyactuable, whereby the valve combines the functions of a remote operated"on-off" flow control valve, having a "fail safe to close" function,with that of a manually operated flow metering or regulating valve forthe control and/or regulation of flowable media.

The flowable media may for example be selected from dry powders,particles, wet or dry granules, liquids, slurries and abrasive oraggressive media whether wet or dry.

One application of the above valves is as an abrasivemetering/controlling valve in apparatus where a rapid response toopening and/or closing instructions is required, e.g. in abrasivecleaning systems such as those described in British Patent No. 2097304and in the present application.

The present invention can advantageously be used in association with theprinciples behind the improved low pressure abrasive cleaning apparatuswhich have in recent years become available. One such apparatus formsthe subject of British Patent No. 2097304.

BRIEF DESCRIPTION OF DRAWINGS

For a greater understanding of the present invention, reference will nowbe made by way of example to the accompanying drawings, in which:

FIG. 1 shows a diagrammatic view of an underwater cleaning or cuttingapparatus;

FIG. 2 shows a modified version of the apparatus of FIG. 1;

FIG. 3 shows a diagrammatic view of alternative underwater cleaning orcutting apparatus;

FIG. 4 shows a modified version of the apparatus of FIG. 3;

FIG. 5 shows a partially cut-away side elevation view (not to scale) ofa mixing zone;

FIG. 6 shows a partially sectional side elevation view of a metering andcontrolling valve;

FIG. 7 shows a section on the line A--A of FIG. 6;

FIG. 8 shows a front view of a handwheel control;

FIG. 9 shows a view taken in the same manner as FIG. 7 with the valvepartly closed; and

FIG. 10 shows (a) a view taken in the same manner as FIG. 7 with thevalve fully closed, and (b) a top view of the valve of FIG. 6 with thevalve fully closed.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring particularly to FIGS. 1 and 2, where like numerals refer tolike parts, an apparatus suitable for undersea abrasive cleaning orcutting work is shown. The apparatus of FIG. 2 includes means foratomising the liquid on entry to the mixing zone, whereas the apparatusof FIG. 1 includes no such atomising means. The components of theabrasive stream are supplied to a mixing zone 1 from main compressed airline 2, grit vessel 3 and water tank 4.

The water tank 4 is connected to a water supply through a ball cockarrangement 5 so as to maintain a constant head of water within thetank.

The compressed air line 2 passes from an external source (not shown) tothe mixing zone 1 via a main on/off manual control valve 6 an automaticmain compressed air regulator 7 (described in more detail below) and anormally-closed control valve 8 which is closed in the depressurised"off" condition.

The grit vessel 3, which is pressurised during operation via compressedair line 2a and a conventional pop-up valve, delivers the abrasiveparticles into the main compressed air line 2 via an accurate meteringtype outlet regulator 9 with setting indicator to the compressedair/abrasive inlet 10 of the mixing zone. A normally-open depressuriservalve 11 is provided to permit recharging of the grit vessel.Alternatively, duplex grit vessels (not shown) and associated valves andpipe work may be employed, connected via transfer valves, to enablecontinuous operation underwater even when replenishing the abrasive.

Water from the tank 4 is fed to a water pump 13 normally operated by acompressed air motor 14 fed from the same main air supply 2, inaccordance with the invention of British Patent No. 2097304, and thencevia supply pipe 15 (through a Y-branch 16 in FIG. 1 and an atomizer 16'in FIG. 2) and into the mixing zone 1 to blend with the compressedair/abrasive mixture to form the abrasive stream.

The pump is preferably of the positive displacement type, either offixed or variable displacement, capable of delivering liquid at flowrates varying from one to ten liters per minute at pressures in excessof 100 psig (7 kg/cm²) above the nozzle ambient pressure.

A flow regulator 26 in the air line feeding the pump air motor enablesthe speed of the motor and pump to be controlled and therefore theliquid flow rate to be adjusted to create the optimum abrasive streamconditions.

The pump may alternatively (not shown) be driven by any other suitablepower source in conjunction with suitable speed and/or flow controls.

The components of the apparatus described above are housed in acontainer (not shown) at or above sea level. The mixing zone 1 has anoutlet 17 leading to a discharge pipe 18 of conventional flexibleconstruction and leads down underwater (shown in dotted lines) to adischarge nozzle 19 operable by a diver at depth, typically at depthsranging for example from 1 meter to 300 meters or even greater than 300meters.

To avoid the danger of syphonic flooding referred to above, theconventional normally-closed valve 8 is provided in the main air supply2 as mentioned above, and a further abrasive resistant bubble tightnormally-closed control valve 20 is provided to close the abrasivedelivery system should the grit vessel pressure fall. Furthermore, aconventional non-return valve 21 (which may alternatively be anormally-closed valve if desired) is provided upstream of the Y-branch16 (in FIG. 1) or the atomizer head 16' (in FIG. 2) in the water supplyline 15.

In a simplified alternative version (indicated schematically in FIG. 3)an automatically closing valve 20 of a type permitting manualincremental abrasive grit flow control may be used, and regulator 9dispensed with. Such a valve is described below by way of example, withreference to FIGS. 6 to 10.

For extra security against leaks or failures, an additional valve 37 maybe fitted between mixing zone 1 and outlet 17, as shown in FIGS. 1, 3and 4. Such a valve 37 may be automatically closing or normally-closedand may be associated with an on-off switch 98 as shown in FIG. 1. Aconventional "non-return" or "check" valve may be provided in line 2(not shown) as protection in case of failure of valve 8.

As will readily be appreciated, for underwater use in order to maintainthe optimum relatively low nozzle pressure of e.g. at most around 100psi (7 kg/cm²) above ambient, the compressed air supply introduced intothe system as motive and control power via inlet pipe 2 must alwaysexceed the ambient pressure at the nozzle 19. A minimum overpressure of25 psi (1.7 kg/cm²) is desirable. Thus, the pressure of the liquidabrasive stream entering the discharge pipe 18 must be proportionallyraised and the abrasive stream flow rate appropriately adjusted to allowfor the greater local hydrostatic pressure encountered at the greateroperational depths. This is suitably achieved by means of a conventionalpressure sensing and transmitting device 22 fitted at the nozzle 19 torespond to changes in local hydrostatic pressure. A pressure gauge 23 isprovided in the apparatus to indicate to surface operators the workingdepth and/or hydrostatic pressure.

The pressure sensing and transmitting device 22 acts by sending a signalto the surface, which can be used to automatically control both a pilotcontrol regulator 24 acting on the regulator 7 controlling the maincompressed air flow, and a pilot control regulator 25 acting on aregulator 26 controlling the compressed air motor 14. An amplifier (notshown) may be used to boost this signal if desired.

Referring particularly to FIG. 1, a differential pilot control switch 99or the like will preferably be used to de-pressure or switch the airsupply or electrical signal from or to the valve actuators 8, 11a, 20aand 37a causing them to close should the pressure of the main compressedair supply entering the system via pipe 2 fall to, or near to, thenozzle ambient pressure, as detected by 22. A "priming" switch 43 isfurnished to initially charge the pressure sensing line, and toreplenish that line in case of leakage. This may be linked to an on-offswitch 27 to ensure closure of all system valves whilst priming.

Manual override regulators 24a and 25a are generally provided inaddition to pilot control regulators 24 and 25 for additional securityor as an alternative should a "manual control only" system be preferred.Regulator isolating valves and non-return valves are also provided.

To ensure bubble tight closure of valve 20 at the very high backpressures obtaining from operation at depth an alternative version maybe used whereby that same hydrostatic pressure obtained via 22 is fed tothe actuator 20a of valve 20 to apply a closing force equal to orgreater than the resultant back pressure acting on the valve internalsto open the valve. A spring may additionally be fitted to assist theclosing force. The valve can be opened as desired to allow grit to bemetered out of vessel 3 by the introduction of mains air onto the"opening" side of valve actuator 20a via a switch and control regulator.Although applicable to either of the apparatus illustrated in FIGS. 2 or4, such a modification has, for clarity, only been incorporated into theillustration of FIG. 4. In that figure, the switch is designated 35 andcontrol regulator 36.

FIGS. 3 and 4 show generally apparatus incorporating pneumatic (oralternatively hydraulic or electrical) controls on the valves 8, 20 and37 and the water pump 13 in a manner which offers very fast valveresponse times and generally improves the security and ease of operatorcontrol, particularly when very deep underwater operations are involved.

In FIGS. 3 and 4, therefore, the main compressed air on/off controlvalve 6 in FIGS. 1 and 2 is replaced by a normally-closed valve 6'arranged to be opened and shut via a system on/off switch 27.Simultaneously as the switch 27 is put to the "on" position all othercontrol switches become "live".

A manual auto-control switch 28 isolates the remote pilot regulators 24and 25 and brings on-stream the manual pilot regulators 24a and 25a, orvice-versa, eliminating the need to close one regulator before operatingthe other every time. A pilot switch 29 acts as the actuator.

A pump on-off switch 30 in conjunction with an auto-closingnormally-closed valve 31 ensures that the pump 13 will stop as soon asthe system switch 27 is put to "off", as well as giving independent pumpcontrol.

For further ease of operation, the apparatus shown in FIGS. 3 and 4incorporate additionally means for assisting the smooth flow of gritfrom the grit vessel 3. Thus, a choke switch 32 together with a pilotoperator 33 and a normally-open by-pass valve 34 provide a means wherebythe valve 8 may be closed during normal operations in the event of afailure of the abrasive flow from vessel 3, to put full inlet airpressure into vessel 3 to assist the grit flow The pilot operator 33would cause valve 34 to open while switch 33 was held in the "Choke"position; and valve 8 closed.

A grit switch 35 and regulator 36 supply air to the "opening" side(underside) of the pneumatic actuator of the normally-closed valve 20(as in FIG. 4), applying a counter pressure to that applied to the"closing" side (topside) of the actuator from 37a (as in FIG. 3) or 22(as in FIG. 4), allowing the valve to open.

The back-up safety shut-off valve 37 may operate in similar fashion tovalve 20, as described above, or may alternatively be as shown in FIG.4, a conventional normally-closed valve A safety interlock may be used,by way of a differential pilot pressure switch 38 (as shown in FIG. 3),or by way of a pilot switch 38 and a pressure switch 39 (as shown inFIG. 4), or the like, whereby no signal is passed to valves 20 and 37(in the case of FIG. 3) or to valve 37 (in the case of FIG. 4) to openuntil the system pressure at (A) is greater than nozzle ambient pressureat 22.

As shown by way of example in FIG. 3, a similar safety interlock 38 Amay be fitted on the compressed air feed line to "on-off" switch 27,preventing any of the system from becoming live unless the inlet airpressure at 2 is greater than the ambient pressure at 22.

FIG. 5 illustrates in more detail the construction of a mixing zone 1,of generally cylindrical form and of substantially the same diameter asthe air line 2. The water supply line (illustrated by arrow X)communicates to a Y-branch 16 which permits the water to enter from theside to impinge on a stream of abrasive particles entering the inletregion 10 from the abrasives hopper 3. No atomising head is present atthe Y-branch 16.

The turbulence and other factors already described cause the particlesto become wetted in accordance with the invention, so that the abrasivestream leaves the mixing zone at the outlet region 17.

It is particularly preferred to use "fail safe to close" valves at 20and/or 37 which have closure springs of sufficient strength to maintainclosure even in the event of a total compressed air supply failure. Thevalve is illustrated in FIGS. 6 to 10 of the accompanying drawings andwill be described with additional reference to FIG. 3, in which such avalve and associated controls are schematically represented.

Referring to FIGS. 6 to 10, the valve is formed by a rubber sleeve `e`which is held firmly in the concentric bores of two halves of an outerhousing, `a` and `b`. The free length of the sleeve is slightly greaterthan the combined length of the two bores in which it is located, sothat it is always under longitudinal compression.

The rubber sleeve `e` is produced with an outer diameter the same orslightly larger than the bore in the housings to produce a mildinterference fit. The ends of the sleeve are flat and square to the boreof the sleeve.

The bottom `o` of each of the housing bores is preferably machinedconically at an angle of between 5° and 15° to the horizontal, so as tocreate a constant "nip" or "set" onto each end of the sleeve when theunit is assembled, the greatest nip being exerted towards the bore ofthe sleeve, and the machined surfaces facing away from the axis of thetube.

Thus, when the sleeve is subjected to either internal or externalforces, or both, the ends of the sleeve will remain sealed against theends of the bores.

For gravity discharge the valve assembly is normally mounted with thebore vertical, as illustrated in FIG. 6, so that housing half `a` wouldbe the lower half and `b` the upper. For pumped or pressure dischargethe assembly may be mounted in any plane.

The inner or joint face of one or both halves is machined out in arectangular shape with rounded corners, as shown in FIG. 7, to asufficient depth to give adequate clearance to two nip heads `c` and`d`, which may suitably be in the form of rollers, when the two housinghalves are bolted together, as shown.

When the sleeve `e` is fitted into the two housing halves the nip headslie to opposite sides of the sleeve.

A pneumatic actuator `k` (designated 20a in FIG. 3), which may be of astandard commercial make (or may alternatively be a conventionalelectrical or hydraulic actuator), is fitted to one side of the housingassembly via support rods `n`, as shown, in such a way that when theactuator is de-activated an actuator shaft `j` can travel (extend) afurther distance than the bore diameter of the rubber sleeve `e`,pushing nip head `d`, which in turn will squeeze the sleeve closed totightly seal the orifice or passage through the sleeve.

A spring `m` is fitted around the actuator shaft `j` against retainer`l` having sufficient force when under compression to completely extendshaft `j` and close the sleeve bore when the actuator is de-activatedagainst the combined working pressure force on the bore of the sleeveand the inherent resistance of the sleeve to compression.

A double acting actuator as shown in FIG. 3, may alternatively be usedto provide additional power to extend the actuator shaft in high workingpressure conditions.

The actuator is so designed that when the power is applied to it toextend or retract the shaft, it will overcome the spring force `m` andfully retract the shaft `j` allowing the sleeve to return to a fullyopen bore, as shown in FIGS. 6 and 7, in which the actuator `k` isactuated or "live".

Nip head `c` is controlled by means of a manually operable handwheel `g`(designated 69 in FIG. 3) which carries a scale `p` and pointer (asshown in FIG. 8), and which rotates a handwheel shaft `f` screw-threadedthrough a shaft support `h` mounted to the side of the valve housing`a`, `b` via support rods `i` extending from the housing. The screwpitch is typically about 1 mm. The shaft support may alternatively (notshown) be integral with, or mounted directly to, the valve housing ifdesired.

The handwheel shaft `f` bears against nip head `c` so that, for aconventional thread, as the handwheel is turned clockwise the shaft `f`will push nip head `c` onto sleeve `e`, and when turned anti-clockwiseit will release pressure from the nip head allowing the sleeve to expandback FIG. 9 illustrates the arrangement after the handwheel has beenturned sufficiently to extend shaft `f` 50% of its normal travel, thusrestricting the size of the orifice through the sleeve through which themedia to be controlled must pass.

In FIG. 10, the handwheel has remained in the same position as in FIG.9, but the actuator has been deactivated and shaft `j` and nip head `d`have been pushed under spring pressure to fully close the sleeveorifice. This position is also shown by the dotted lines in FIG. 6.

The preset partial closure of sleeve `e` can be varied from fully openedto fully closed by ensuring a sufficient length of thread on shaft `f`.The degree of closure can be shown to the operating personnel either byhaving, attached to the housing assembly, a linear indicator alignedagainst a mark or markings on the shaft `f` (not shown), or asillustrated in FIG. 10, a gravity dial indicator may be used, where onerotation of the handwheel or handle moves the pointer one graduation onthe dial, which equals one pitch length of the thread.

Thus the valve aperture may be infinitely varied to give accurate flowcontrol, the valve will close bubble tight automatically on switch offor power failure, and it will open again repeatedly to the presetaperture on switching on again.

For high pressure service and for handling dangerous substances, glandsor seals may be fitted where the two shafts `j` and `f` pass through thehousings.

Pressure gauges, filters F, lubricators L, valves, safety relief valvesetc. may generally provided at suitable places in the apparatus inconventional manner. Where not specifically described above, manualcontrols M for the automatic valves are provided in conventional manner.Hot water or steam jackets, e.g. for the grit vessel, water tank, mixingzone and water pump and motor, may also be present to improveperformance and water flow and to prevent icing up in cold weather.

Industrial Applicability

It is found that the safety and efficiency of the apparatus of theinvention is extremely high compared to any previously known systems,and that less and cheaper abrasive can be used for a given operationthan hitherto. It is also found that the very low vibration level at thenozzle and the very low reverse thrust makes the apparatus very suitablefor use with remote-operated vehicles. In particular, using theapparatus of the invention cleaning rates can be improved by factors ofbetween 8:1 and 15:1 compared to prior systems, with abrasiveconsumption reduced by 15 to 30 times compared to prior high pressuresystems (depending on factors such as operating depth and the hardnessand thickness of the dirt or coating to be removed or cut).

Some of the potential benefits of the present invention in its variousaspects, when embodied in an underwater abrasive cleaning or cuttingapparatus, may be summarised as follows:

(a) it allows the motive power for propelling the cleaning or cuttingmedium against the object to be cleaned or cut to be provided bycompressed air or gas;

(b) it allows the abrasive mixture to be discharged from a single outletvia a single flexible hose or pipe leading from the mixing zone to thenozzle;

(c) it allows an air compressor to be sued either external to or insidethe apparatus housing;

(d) it allows the relative proportions of the ingredients of theabrasive mix to be varied to suit the nature of the cleaning or cuttingtask;

(e) it allows the discharge pressure and velocity of the abrasive mediumto be adjusted manually to suit the nature of the task and the depth ofunderwater operation;

(f) it allows the hydrostatic pressure at the nozzle to be monitored;

(g) it allows either or both of (i) the proportions of the ingredientsof the abrasive mix and (ii) the discharge pressure and velocity of themix to be automatically adjusted to suit the ambient pressure at thenozzle;

(h) it allows the discharge pressure of the abrasive medium at thenozzle to be maintained up to about 100 psig (7 kg/cm²), normally about30 to 50 psig (2.1 to 3.5 kg/cm²) above the ambient pressure at thenozzle;

(i) it allows monitoring and control devices to be operated manually,pneumatically, hydraulically or electronically;

(j) it allows the nozzle to be held and manipulated by a diver or aremote operated vehicle with equal facility without the need for thrustor vibration compensators;

(k) it allows air to be fed from a compressor or external power source,liquid to be fed from a pump and abrasive particles to be fed from apressurised container, with each inlet into the mixing zone and theoutlet from the mixing zone having independent manually or automaticallyactuable valve means to isolate the respective inlet or outlet;

(l) it allows the isolating valves mentioned in (k) to be controlled insuch a way that they will automatically close in the event that thesystem discharge pressure should fall below the nozzle ambient pressure,thus preventing a back-flow of wet air or water back into the apparatuswhen either the flow or pressure of propellant is insufficient toovercome the ambient pressure, or when the apparatus is deactivated andde-pressurised; and

(m) it enables products such as concrete, which is commonly applied tounderwater oil and gas pipelines as an all round protective casing some3" to 4" (75 mm to 100 mm) thick, and known as "weight coating", to becut through safely, efficiently, and leaving a relatively clean,unbroken, edge suitable for allowing the removal of complete sections ofsuch casing, in one or more pieces as required, using manual ormechanical means.

I claim:
 1. Apparatus for underwater abrasive cleaning and/or cutting,comprising:a mixing zone for preparing an abrasive mixture comprisingabrasive particles, air and a liquid; means for controlledly supplyingthe abrasive particles, air under pressure, and liquid to the mixingzone in such a way that a resultant abrasive stream includes abrasiveparticles substantially surface-wetted by the liquid and entrained inair or an air/liquid mist as an abrasive carrier; an outlet nozzle fordirecting the abrasive stream at an underwater surface to be cleanedand/or cut; a pipeline connecting the mixing zone to the outlet nozzlefor conveying the abrasive stream to the nozzle; and means for adjustingthe flow rates of the abrasive particles, air and liquid and the mixingzone pressure, relative to each other, depending one the working depthof the nozzle underwater, so as to discharge an abrasive stream ofcomposition as set forth above through the nozzle at a pressure lessthan about 100 psig (7 kg/cm²) above the ambient hydrostatic pressure atthe nozzle.
 2. Apparatus according to claim 1, wherein the mixing zoneis provided with a first inlet port for receiving a stream of aircarrying abrasive particles and a second inlet port, downstream of thefirst inlet port, for receiving a supply of liquid.
 3. Apparatusaccording to claim 2, wherein the second inlet port is arranged so thatthe liquid passing therethrough impinges on the stream of air carryingabrasive particles in such a way that the liquid breaks into droplets ofa size generally similar to, or somewhat larger than, the size of theabrasive particles.
 4. Apparatus according to claim 2, wherein the meansfor supplying the abrasive particles and air to the mixing zone comprisea pressurized air lien and a bypass line leaving the air lien, enteringa pressurized container for abrasive particles, leaving the containercarrying entrained abrasive particles and rejoining the air lineupstream of the point of supply of the liquid.
 5. Apparatus according toclaim 1, wherein the means for supplying the liquid to the mixing zonecomprise a pneumatically powered pump.
 6. Apparatus according to claim1, wherein valve means are provided to control the flow of at least oneof the abrasive particles, the air, the liquid and the abrasive mixtureprepared therefrom.
 7. Apparatus according to claim 1 wherein valvemeans are provided upstream and/or downstream of the mixing zoneactuable to restrict or prevent flooding of surface apparatus due toreverse-flow of abrasive mixture in the pipeline.
 8. Apparatus accordingto claim 6 or claim 7, wherein at least one of the valve means isactuable in response to local hydrostatic pressure at the nozzle. 9.Apparatus according to claim 6 or claim 7, wherein at least one of thevalve means has an adjustable extent of closure and may be pre-set toprovide a desired degree of closure when actuated.
 10. Apparatusaccording to claim 6 or claim 7, wherein at least one of the valve meanscomprises a resilient tube snugly retained under longitudinalcompression within a chamber and seated therein by expansion againstabutments provided int he chamber, the arrangement being such that therespective flowable medium may pass through the tube in use and meansbeing proceed for wholly or partially constricting the tube, wherein theabutments in the chamber are so shaped that at least part of the surfaceagainst which the tube is seated faces away from the axis of the tube.11. Apparatus according to claim 1, wherein the pipeline is a singleflexible hose or pipe.
 12. A method of underwater abrasive cleaningand/or cutting, wherein an abrasive stream comprising a mixture ofabrasive particles, air and a liquid and including abrasive particlessubstantially surface-wetted by the liquid and entrained in air or anair/liquid mist as an abrasive carrier is initially prepared in apressurized mixing one and subsequently directed, at a pressure lessthan about 100 psig (7 kg/cm²) above the ambient hydrostatic pressure,at an underwater surface to be cleaned and/or cut, the method furtherincluding manually and/or automatically adjusting the flow rates of theabrasive particles, air and liquid and the mixing zone pressure,relative to each other, depending one the working depth of cleaningand/or cutting so as to provide the abrasive stream composition andpressure as set forth above.
 13. A method according to claim 12, whereinfrom 80 to 100% of the liquid in the abrasive stream goes tosubstantially encapsulating at least a majority of the abrasiveparticles and the remainder, if any, of the liquid goes to form theair/liquid mist.
 14. A method according to claim 12, wherein from 90 to95% of the liquid in the abrasive stream goes to substantiallyencapsulating at least a majority of the abrasive particles and 5 to 10%of the liquid goes to form the air/liquid mist.
 15. A method accordingto claim 12, wherein the abrasive stream is prepared by allowing asupply of liquid to impinge on a stream of air carrying abrasiveparticles within a pressurized mixing zone, in such a way that theliquid breaks into droplets of a size generally similar to, or somewhatlarger than, the size of the abrasive particles.
 16. A method accordingto claim 12, wherein the motive power for propelling the abrasivemixture against a surface to be cleaned and/or cut is provided bycompressed air.